/Engine/Source/Runtime/Analytics/Analytics/Public/Interfaces/IAnalyticsProvider.h
Set the UserID for use with analytics. Some providers require a unique ID to be provided when supplying events, and some providers create their own. If you are using a provider that requires you to supply the ID, use this method to set it.
void SetUserID ( const [FString](API\Runtime\Core\Containers\FString) & InUserID )